Some evidence supports the hypothesis that the mutation rate at a particular site is influenced by neighboring base pairs. I have sought to modify the reversion and conversion frequencies of rII nonsense codons by inserting nearby temperature-sensitive rII lesions. The spontaneous and 2-aminopurine induced reversion rate of an amber mutation is decreased threefold by inserting a ts mutation 10 to 30 base pairs away. Site-specific mutation rates will be measured using the reversion and conversion pathways of the corresponding ochre mutation.